(CNN) If you are a man who has hesitated to get the Covid-19 vaccine due to concerns spread on social media that the vaccine may harm fertility, take heart.
Sperm count and quality did not drop in healthy young men after receiving a first or second dose of the Pfizer or Moderna vaccine, according to a new study published Thursday in JAMA.
"We now have evidence that should reassure you that the risk of immunization compromising your sperm count is extremely low," said Dr. David Cohen, co-medical director of the Institute for Human Reproduction in Chicago, who was not involved in the study. Cohen was the co-author of a systematic review that found the novel coronavirus is not sexually transmitted.
"This is reassuring data which suggests that sperm quality is not significantly altered by receiving two doses of one of the new mRNA vaccines for COVID-19," said Allan Pacey, professor of andrology at the University of Sheffield in the UK, who also was not involved in the study.
"I hope this provides some reassurance to any men who may be concerned about their fertility if they accept one of these types of vaccines," said Pacey, who is also editor-in-chief of the journal Human Fertility.
The study used semen samples from 45 men between the ages of 25 and 31, who were pre-screened to make sure they had no fertility issues. Samples were taken before the first shot of an mRNA vaccine, then 70 days after the second dose. Semen was then examined to determine sperm volume, concentration, motility and total sperm count.
"We found no changes in sperm parameters in the young healthy men that we studied who received both doses of mRNA vaccine," said study author Dr. Ranjith Ramasamy, director of male reproductive medicine and surgery at the University of Miami Health System.
The study did not test the Johnson & Johnson or AstraZeneca vaccines, which are not based on mRNA platforms.
"However, we think the mechanism of how these vaccines work is all fairly similar despite different genetic material, so based on biology, we don't think there should be anything different with the other two vaccines," Ramasamy said.
Additional larger studies with men of different ages are needed to affirm the study's results, Pacey said.
While the vaccine had no impact on male sperm in the study, the virus itself could be harmful to the male reproductive tract, according to a small study published in January.
Compared to healthy men without Covid-19, the study found a significant increase in inflammation and oxidative stress in sperm cells belonging to men with Covid-19. Their sperm concentration, mobility and shape were also negatively impacted by the virus.
The differences grew with the severity of the sickness, the study found.
It's not surprising that SARS-CoV-2, the virus that causes Covid-19, might impact the male reproductive system, because ACE2 receptors, the "same receptors which the virus uses to gain access to the tissues of the lung, are also found in the testicles," Pacey said.
Pacey has reviewed over a dozen studies published on the topic, and he told CNN in January he had concluded that "any measurable effect of coronavirus on male fertility was probably only slight and temporary."
Medications used to treat Covid-19, fevers, obesity and many other factors can also impact sperm count and quality, so larger studies would need to be done to ensure that it was the virus causing the effect, Pacey said.
"We know that some viruses like Zika or Ebola can remain in the testicles for a long period of time -- long after the man has recovered from them. So we need to study men in the longer term to know if this is also the case for Covid-19," he said.